Wildcard domains: A domain name beginning with '@', such as
@example.com, is called a wildcard domain. In our system this
represents all unknown addresses in the domain.

To keep out dictionary attacks and prevent
excessive machine load, email arriving for a wildcard domain,
i.e., for unknown addresses in a domain, will always be subjected
to a high degree of SMTP filtering and this cannot be switched off.
You may create specific email addresses
in your domain and separately adjust the SMTP filtering for each one.

References: For more information about greylisting and black lists, please see: